1. Field of the Invention
The present invention relates to a micro-actuator operated by electrostatic force and a manufacturing method thereof, and more particularly, to a micro-actuator driven in a see-saw motion in an axial direction and a manufacturing method thereof.
2. Description of the Related Art
U.S. Pat. No. 5,025,346 discloses a micro-actuator operated by the electrostatic force induced by the structure of a comb-type electrode. The micro-actuator disclosed in U.S. Pat. No. 5,025,346 has a structure with a plurality of moving comb-type electrodes and a plurality of fixed comb-type electrodes arranged alternately on a moving structure and a fixed structure, respectively. The moving structure is suspended by a surrounding supporting structure and the suspended structure is driven at a resonant frequency in the horizontal direction.
To drive in the direction of more than 1 axis such as the x-axis and the y-axis, if driving is in one direction along one axis, more than 3 electrodes are needed in the driving part, and if driving is in both directions along one axis, at least 5 electrodes are needed. U.S. Pat. No. 5,536,988 discloses an actuator that is fabricated using a modified single crystal reactive etching and metalization process which incorporates an isolation process utilizing thermal oxidation of the device.
In the prior micro-actuator, the driving comb-type electrodes are arranged in a direction parallel to the plane of the stage of a moving body or a moving structure, and the fixed comb-type electrodes are arranged on a fixed structure, alternating with the driving comb-type electrodes and parallel to the plane of the stage in the same way as the driving comb-type electrodes.
Since the comb-type electrodes are arranged in the neighborhood of the stage in the prior micro-actuator as described above, the total size is expansive compared to the stage or the moving structure. The prior micro-actuator has a structure driven in the direction of an axis or two axes (x,y-axis) parallel to the plane of the stage or the moving structure, and the application is thus limited.
Korean patent application no. 98-37315 and Korean patent application no. 99-20488 that claims the priority based on the latter application disclose a micro-actuator with a see-saw driving method in one axial direction. The applications disclose a micro-actuator applicable to a optical disk drive. This micro-actuator may be applied to various fields as a optical scanner as well. However, this see-saw driving micro-actuator has a stage back driving structure where the driving comb-type electrodes and the fixed driving comb-type electrodes are arranged at the back of the stage and on the top of the plate corresponding to the back, and since the top and bottom structures are manufactured using two substrates and these are joined together afterward, the manufacture is not easy. Particularly, it is difficult to join the top and bottom structures if foreign materials exist on at least one of the driving comb-type electrodes and the fixed comb-type electrodes arranged in each structure. Therefore, there is a need to provide a structure where the top and bottom structures can be processed separately and joined successfully, and a method for its manufacture.
It is therefore an object of the present invention to provide a micro-actuator which is easy to manufacture and has a high yield, and it is a second object of the present invention to provide a method of manufacturing the same.
To achieve the first object, the micro-actuator of the present invention comprises a base plate on which a predetermined pattern of signal lines is formed, a plurality of fixed comb-type electrodes that are arranged on the base plate and extend in a direction perpendicular to the base plate, a stage capable of a see-saw motion that is arranged at a predetermined height from the top of the base plate, a plurality of driving comb-type electrodes which are formed parallel to each other on the bottom of the stage and have ends that extend between the fixed comb-type electrodes, a torsion bar with a predetermined length that is arranged at both ends of the stage forming one body with the stage in order to enable the see-saw motion of the stage, a first frame layer connected to both ends of the torsion bar, a second frame layer that is positioned below the first frame layer, thus forming a layered structure with the first frame layer, and a metal eutectic bonding layer formed between the first and second frame layers to bond them together.
In the micro-actuator of the present invention, the first frame layer, the torsion bar, the stage, and the driving comb-type electrode are preferably formed integrally.
The first frame layer has the shape of a rectangular border that surrounds the stage. Between the first frame layer and the stage, a separate region of a predetermined width exists, which the torsion bar preferably crosses.
The fixed comb-type electrodes are formed on an electrode base that is arranged on the base plate. The electrode base, the fixed comb-type electrodes, and the second frame layer are made preferably from the same material plate.
The fixed comb-type electrodes are higher than the second frame layer, and thus the front ends of the fixed comb-type electrodes are positioned higher than the top of the second frame layer. Also the front ends of the driving comb-type electrodes and the first frame layer are preferably on a common plane.
The metal eutectic bonding layer of the present invention is composed of a plurality of metal layers, among which the middle metal layer is preferably a layer plated with Au/Sn alloy.
To achieve the second object, a method for manufacturing the micro-actuator of the present invention comprises the steps of forming a top structure by etching both sides of a first plate, the top structure comprising a stage, a plurality of comb-type electrodes formed on the bottom of the stage, a torsion bar positioned in the middle of both edges facing the stage, and a first frame layer of a predetermined height supporting the torsion bar, forming a bottom structure by etching both sides of a second plate, the bottom structure comprising a base plate, a second frame layer formed on the base plate and having a predetermined height corresponding to the first frame layer height, and a plurality of fixed comb-type electrodes formed on the base plate, and joining the top and bottom structure to form one body by forming a eutectic bonding layer between the first frame layer and the second frame layer, and superimposing the driving and fixed comb-type electrodes such that the extensions of the driving comb-type electrodes alternate with the extensions of the fixed comb-type electrodes.
In the method for manufacturing a micro-actuator of the present invention, preferably, the step of forming the top structure further comprises the steps of forming a top separate region with a predetermined width and depth corresponding to the space between the stage and the first frame layer, forming a top metal layer on the region corresponding to the first frame layer, and forming the driving comb-type electrodes with a predetermined height on the bottom of the stage, while the separate region is penetrated by etching the bottom of the first plate with a predetermined pattern.
It is also preferable that the step of forming the bottom structure further comprises the steps of forming signal lines with a predetermined pattern corresponding to the constituent elements, forming a bottom separate region with a predetermined width and depth corresponding to the space between the second frame layer and the fixed comb-type electrodes, joining the bottom of the second plate to the top of the base plate, etching the region corresponding to the second frame layer on top of the second plate to a predetermined depth, forming a bottom metal layer at the etched part of the second plate, forming a mask layer on the region corresponding to the second frame layer and the fixed comb-type electrodes on top of the second plate, and forming the fixed comb-type electrode with a predetermined height inside of the bottom separate region, while the bottom separate region is penetrated by etching the region that is not covered by the mask layer to a predetermined depth.
In addition, in the method of manufacturing a micro-actuator of the present invention, it is preferable that the step of forming the top metal layer further comprises the steps of forming a metal seed layer at the first plate and forming a metal eutectic bonding layer by a plating method on the seed layer.
Also, it is preferable that the step of joining the top and bottom structures into one body is a step of performing the metal eutectic bonding at a predetermined temperature and pressure in contact in order to join the first frame layer of the top structure to the second frame layer of the bottom structure, and more specifically with the top metal layer of the first frame layer of the top structure and the bottom metal layer of the second frame layer of the bottom structure in contact.